Mac and iPad iCloud and iTunes photo syncs question

No. Unless it has changed very recently, the very first thing that will happen if you turn on iCloud Photo Library on a device that contains photos that have been synced to it via iTunes is that you will get a warning that photos will be deleted from that device.

This is stated (although not quite as clearly as I think it should be) in a section discussing "missing" photos, in the excerpt below from Get help with your iCloud Photo Library - Apple Support

Any photos or videos you copy to your device through iTunes are not copied in iCloud Photo Library.

freddedotme wrote:

He has them synced to iCloud though.

I understand that he has photos on his Mac already synced to iCloud and wants to 1) sync them to his iPad directly from his Mac (via iTunes) and then 2) turn on iCloud Photo Library on the iPad. I'm guessing he thinks that might be faster than just turning on iCloud Photo Library on the iPad and letting them sync via iCloud.

If that is what he is trying to do (and I could be misinterpreting), after he syncs the photos to the iPad via iTunes and then turns on iCloud Photo Library on the iPad, the photos synced via iTunes will be deleted and then iCloud photos will be synced. All of the correct photos will end up on both devices but not in the way I think he plans.

Yes that works, any device you connect to iCloud will have access to the same files.

No. When you turn on iCloud Photo Library on an iOS device, any photos synced to the device via iTunes are deleted.

He has them synced to iCloud though.